Geo chem i cal and fractal anal y sis of en claves in the Dehe-Bala in tru sion, (North west ern Iran): a new con cept in the in ter pre ta tion of the crust-man tle in ter ac tion pro cess Gharamohammadi, Z., Kananian, A., Eliassi, M., 2019. Geo chem i cal and fractal anal y sis of en claves in the Dehe-Bala in trusion, (North west ern Iran): a new con cept in the in ter pre ta tion of the crust-man tle in ter ac tion pro cess. Geo log i cal Quar terly, 63 (3): 505-521, doi: 10.7306/gq.1481 As so ci ate ed i tor: Wojciech Granoszewski The Dehe-Bala in tru sion is one of the re mark able in tru sions of granodiorite rocks with I-type af fin ity and abun dant mafic microgranular en claves (MMEs) in the Buin Zahra area, Qazvin, Iran. The MMEs, com posed of diorite and quartz-monzodiorites, are hap haz ardly wide spread in the granodiorites. The Dehe-Bala Granodiorites (DBG) usu ally are char ac ter ized by high con tents of SiO 2 (64.2-66.9), Na 2 O (3-3.23), K 2 O (3.49-4), Mg# 4.84 and Th/Ta ra tio (»7.9). In compar i son to the DBG, the MMEs can be dis tin guished by their lower value of SiO 2 (52.8-58.2), K 2 O (1.4-3.8) and higher Mg# (0.4-0.46). All these char ac ter is tics show a dif fer ent com po si tion of the DBG and MMEs, more im por tantly, can ar gue in favor of a magma mix ing/min gling or i gin in the DBG. The en rich ment in to tal REEs and HFSEs in the MMEs clearly re flects a marked diffusional pro cess from the fel sic to mafic magma that could have been achieved by chem i cal ex change dur ing the magma mix ing/min gling pro cess. The fractal di men sions (D box ) of MMEs dif fer from 1.14 to 1.29 with the high est fre quency at 1.29. The tex tural het er o ge ne ity and geo chem i cal fea tures com bined with high D box val ues in the MMEs com pared with the DBG show lower de grees of mix ing/min gling be tween man tle-de rived mafic and lower crust-de rived fel sic mag mas.Fig . 1A -sim pli fied map of tec tonic zones of the Zagros Orogenic Belt, Urumieh-Dokhtar Mag matic As sem blage (UDMA), Sanandaj-Sirjan Zone (SSZ), Zagros Fault Thrust Belt (ZFTB); B -sim pli fied geo log i cal map of the DBG in south -west Buin Zahra (af ter Eghlimi, 2000)
<p>A Cretaceous paleo-accretionary wedge (the Ashin Complex) now exposed along the Zagros suture zone in southern Iran exhibits mafic and metapelitic lithologies. Field, geochemical and petrological observations point to a high-temperature event that gave rise to the formation of peritectic (trondhjemitic) melts associated with restitic garnet-bearing amphibolites in the structurally highest sliver of the Ashin Complex. SHRIMP U-Pb zircon dating of grains crystallized in trondhjemitic leucosomes yields a <sup>206</sup>Pb/<sup>238</sup>U weighted mean age of 104 &#177;1 Ma, interpreted as the peak temperature event, which occurred in the amphibolite facies (c. 640-650&#176;C at 1.1-1.3 GPa), based on thermodynamic modeling. Rutile crystals from several leucosomes yield Zr-in-rutile temperatures between 580-640&#176;C and LA-ICP-MS U/Pb ages of 87-94 Ma. This rutile generation may be related to the observed static formation of Na-clinopyroxene and Si-rich phengite rims, as well as the growth of lawsonite in late fractures. The latter paragenetic sequence has been previously interpreted as reflecting a long-term isobaric cooling that occurred at least until the end of the Cretaceous (ages in Angiboust et al., 2016).</p><p>While the latter observations point to a long-term cooling of the Zagros subduction thermal gradient down to 7&#176;C/km during late Cretaceous times, this first report of an earlier melting event in the Zagros paleo-accretionary wedge indicates an abnormally high thermal gradient of 17-20&#176;C/km. GPLATES paleogeographic reconstructions of the Tethyan realm evolution during Cretaceous times reveal the presence of a spreading ridge jump followed by the subduction of the formerly active ridge-segment between 105-115 Ma, which possibly left an imprint marked by the unusually hot gradient seen in Ashin amphibolites. The model further predicts the subduction of progressively aging oceanic lithosphere, possibly explaining the observed cooling of the subduction thermal regime.</p>
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